Ever since man opened eyes he had the desire to reproduce reality in the form of drawings and pictures. The invention of photography created a new form of life. There is no home or office, road or a subway that is not filled with pictures. All based on the concepts of a photograph. Everybody has accepted that pictures are projection of reality onto a screen. However all such pictures are based on a single viewpoint. Naturally for a being who looks at the reality with two eyes such pictures cannot reflect the reality as he sees it. Re-creation of reality i.e. the three dimensional pictures has been a continuing subject for inventors in the past hundred and fifty years. The original stereoscopes handled this by providing two different images in front of each eye thereby giving the viewer the feel of seeing different images with each eye and hence the illusion of three dimensionality of the picture. The requirement of having to produce a right eye and a left eye picture and the desirability to have only one screen has given rise to many ingenious methods and instruments. These days the most popular technique is projecting two pictures onto a screen in two different colours and then requiring the viewer to wear glasses with complimentary coloured lenses on each eye. The effect is that each eye would see only one of the pictures, hence giving the viewer the required illusion. This technique is developed to provide means of seeing in colour (for examples see The British Journal of photography 17th Jan. 1991, page 18 or U.S. Pat. No 3,712,199 of 1973 or U.S. Pat. No 4,480,893 of 1984). A similar version of this technique is to use polarised lenses and polarised pictures instead of using complimentary colours. Another old idea which keeps getting reborn is lenticular screens (see for example European patent no EP 0384768 A3 of 1990). The clear advantage of such method is that the viewer does not need to wear special tools for viewing. The desirability of not wearing special tools has in itself created other lines of inventiveness. One such method is to place the right and left eye views side by side, however on the reverse side. The viewer by looking at the two pictures in cross eyed fashion can match the two pictures in a different distance than the screen and gets the three dimensionality illusion. The disadvantage of such method is that it puts too much strain on the eye muscles and the useful part of the view becomes considerably smaller than the screen. Some effort has been put on trying to mix such left and right views (see for example U.S. Pat. No. 4,135,502 of 1992). In this method repeated parallel patterns are created so that the right eye view shares some of the items of the left eye view. When the viewer makes himself cross eyed, distinct patterns are matched as if they were the same item and the viewer sees the pattern in a different position than the screen. This method is of limited use as it can only be used for artificial repeated patterns.
By far the most interesting technique for producing a three dimensional image which does not require special viewing instrument has been the idea of holograms. A hologram by creating wave-fronts produces the illusion of the object itself. With more complicated holograms e.g. a composite hologram, you may even walk around the hologram and view the object from all sides. Holograms however have an inherent problem when it comes to colours. The complication of producing such images and the technical requirements are so demanding that despite much research, all attempts to put movement and stability to the pictures are not properly achieved and there is not yet an acceptable commercial production available.
The basis of all these developments is creation of two different images one for each eye essentially the same as the original stereoscopy. These pictures are merged into each other by the brain and the brain, by comparison of the distinguishing features of different objects in the picture, recognises their relative distances and hence submits to the illusion of three dimensionality. It is rather obvious but necessary to mention that for any of the above techniques, it is necessary that the viewer has at least two eyes to see these pictures. Sensations created by three dimensional pictures are rather complex subject with various effects. When talking about a three dimensional image, one does not mean that the image itself is of three dimensions. It means the viewer believes that the image formed in his brain is originated from real three dimensional objects. The brain, being the complex machine that it is, recognises the three dimensionality of the view in several different ways. These can be classified as follows;
1. Duality: Due to the fact that the eyes are approximately 6.5 cm apart , the images recorded by each eye are different. The location of each object in the two different images which are received by each eye are analysed by the brain thereby one recognises the relative distances of the objects from each other and the view point. PA1 2. Head movements: Movement of the head back & forth, left & right, up & down, rotation along the axis of the top of the nose, rotation along the neck axis, all cause the images in the eyes to vary. By analysing the relative location of objects in these images the brain can analyse their distances. PA1 3. Eye Movements: The movement of the eyeballs within the sockets to concentrate on objects in different distances (cross-eyed for near and parallel for distant objects) and also attempts to focus on an object put different pressures on the muscles of the eye. These pressures are analysed by the brain to recognize their distances.
Essentially it is the comparison of two different situations and their relative distinction which creates the three dimensional effect. Some of these distinctions are instantaneous e.g. seeing two images with each eye at the same time. However some of these relative comparisons are only achieved during time. For example when one walks in front of an object whilst looking at it, the relative position of the object to its background changes. By comparison of the images which have been received by the eyes at two different points in time, one is able to note that the object is nearer to him than its background. Hence some three dimensionality effects can only be revealed to the brain by getting into the fourth dimension, i.e. time. It is the combination of all the above which creates a complete three dimensional concept in the brain. Sometimes the word paralex is used to summarise the effects of three dimensionality. Such a concept is by no means sufficient to explore all that happens in the human brain and it only serves to confuse the subject. Most of the above methods are said to have horizontal paralex but no vertical paralex. What it means is that the picture remains the same if there were a vertical head movement. However if there were a horizontal head movement, in all the above techniques, other than holograms, the picture still remains essentially the same. Instead of referring to horizontal paralex, one should state that these techniques allow limited eye movement within their socket but with no allowance for variations due to head movements.
The recognition of three dimensionality due to the head and eye movements are rather obvious and are rarely explored (see R. Kingslake, "Applied optics and optical Engineering", 1965, Academic Press New York and London, Vol II, chapter 2). What is more complicated is the duality. That is what the brain does with the two distinct images obtained by each eye from the same view at an instant in time. Some are of the belief that the brain has the capability to match every object in view at all times and hence creates a duplicate three dimensional space in the imagination. Some even confuse the effort exerted by the brain to see the view in three dimensions as three dimensional sensation. For example in GB 210472 Stanley, explains a method of one picture stereoscope where the same picture is shown to the viewer for both eye views. True that the brain has to exert the same type of effort to match the two pictures as would have done when looking at reality with two eyes but it is not the physical effort put by the brain which creates the third dimension. It is the information conveyed to the brain which is important. How can two pictures which are the same give the brain more information than they contained originally.